The aim of the proposed study is to investigate the role of the renin-angiotensin-aldosterone system in the maintenance of fetal blood pressure and placental perfusion in the presence of intrauterine hypoxic and hypovolemic stress. This data may contribute to the understanding of intrauterine and perinatal hypoxia which is a leading cause of infant mortality and morbidity. Fetal carotid pressure, umbilical artery blood flow and plasma angiotensin II levels will be measured in unanesthetized, chronically instrumented fetal lambs (105-115 days gestation) subjected to hypoxia and hypovolemia before and after competitive blockade of angiotensin II and of angiotensin I conversion. In a series of 25 ewes, fetal hypoxia will be produced by having the unanesthetized ewe breathe 10% O2, 2% CO2 and 88% N2. The maternal and fetal blood gases will be monitored. The effects of intravenous administration of saralasin (80 microgram/Kg) and SQ20881 (2 mg/Kg) on the fetal blood pressure, umbilical artery blood flow and arterial blood gases will be observed and compared with the measurement of the same parameters prior to blockade of the renin-angiotensin system. In another series of 25 ewes, fetal hypovolemia will be produced by removal of 10 cc/kg of blood from the fetus. The effects of this procedure on the fetal carotid pressure, umbilical artery blood flow, and plasma angiotensin II levels will be determined before and after administration of saralasin and SQ20881 at the same dosages.